1. Field of the Invention
The present invention relates to a resettable dual pinned spin valve sensor with thermal stability and demagnetizing fields balanced by sense current and ferromagnetic fields and, more particularly, to such a sensor which has a triple antiparallel (AP) pinned layer structure on one side of a free layer and a single pinned layer on the other side of the free layer.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor are a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Readback asymmetry is defined as             V      1        -          V      2            max    ⁢          xe2x80x83        ⁢          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in some applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnitizing (demag) field HD from the pinned layer, a sense current field H1 from all conductive layers of the spin valve except the free layer, a net image current field H1M from the first and second shield layers. The strongest magnetic force on the free layer structure is the sense current field H1. In an exemplary bottom spin valve sensor where the free layer is closer to the second gap layer than it is to the first gap layer the majority of the conductive layers are below the free layer structure between the free layer structure and the first gap layer. The amount of conductive material in this region is further increased if the pinning layer is metal, such as platinum manganese (PtMn), instead of an oxide, such as nickel oxide (NiO). When the sense current is conducted through the sensor the conductive layers below the free layer structure cause a sense current field on the free layer structure which is minimally counterbalanced by a typical cap layer made of tantalum (Ta) on top of the free layer structure.
A dual spin valve sensor may be employed for increasing the magnetoresistive coefficient dr/R of a read head. In a dual spin valve sensor first and second pinned layers are employed with a first spacer layer between the first pinned layer and the free layer and a second spacer layer located between the second pinned and the free layer. With this arrangement the spin valve effect is additive on each side of the free layer to increase the magnetoresistive coefficient dr/R of the read head.
It should be noted that the magnetic moments of the pinned layers on each side of the free layer must be parallel with respect to one another in order for the spin valve effects on each side of the free layer to be additive. With this arrangement the demagnetizing fields from the pinned layer on each side of the free layer will be additive and that the ferromagnetic coupling fields, due to the interfacing of the AP pinned layers with the spacer layer, will likewise be additive. Since the net demagnetizing field and the net ferromagnetic coupling field are additive the total of these fields must be counterbalanced by the net sense current field due to the conductive layers on each side of the free layer. Even though the net sense current field is the largest field acting on the free layer, it may not be sufficient to counterbalance both of the net demagnetizing field and the net ferromagnetic coupling field in order to obtain read signal symmetry. Help can be obtained for the net sense current field by offsetting the free layer with respect to the first and second shields so that a net image current field is parallel to the sense current field. However, it is preferable to avoid this offset since one of the read gap layers may have to be thicker in order to properly insulate the spin valve sensor from one or both of the shield layers and/or insulate lead layers to the sensor from the second shield layer. Another alternative is to reduce the net demag field by employing AP pinned layer structures in lieu of pinned layers in a dual spin valve sensor. An AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers. The first and second AP pinned layers have magnetic moments which are antiparallel with respect to one another because of the strong antiferromagnetic coupling therebetween. Because of a partial flux closure between the first and second AP pinned layers of each of the first and second AP pinned layer structures, each AP pinned layer structure exerts only a small demagnetizing field on the free layer. The AP pinned layer structure is fully described in commonly assigned U. S. Pat. No. 5,465,185 which is incorporated by reference herein. An AP pinned layer structure on each side of the free layer may not be practical where a thin read gap is desired for increasing the linear read bit density of the read head. An AP pinned layer structure has more layers than a pinned layer which increases the thickness of the spin valve sensor thereby increasing the thickness of the read gap. Accordingly, there is a strong-felt need to obtain more flexibility in counterbalancing the fields exerted on the free layer in a dual spin valve sensor.
The present invention provides more flexibility in counterbalancing the fields acting on the free layer by providing a triple AP pinned layer structure on one side of the free layer and a simple pinned layer on the other side of the free layer. The triple AP pinned layer structure has ferromagnetic first, second and third AP pinned layers and first and second AP coupling layers with the second AP pinned layer being located between the first and second AP coupling layers and the first and second AP coupling layers being located between the first and third AP pinned layers. Accordingly, the first AP pinned layer interfaces and is exchange coupled to the first pinning layer and the third AP pinning layer interfaces the first spacer layer. In a first embodiment of the invention the AP pinned layer structure exerts no demagnetizing field on the free layer. This is accomplished by providing the second AP pinned layer with a magnetic thickness which is equal to a total of the magnetic thicknesses of the first and third AP pinned layers. In this embodiment the only demagnetizing field exerted on the free layer is due to the pinned layer on the other side of the free layer. In this embodiment a setting of the magnetic spins of the first and second pinning layers is accomplished by an exterior magnetic field in the presence of heat which is sufficient to free the magnetic spins so that they can be moved by the exterior magnetic field. In a second embodiment the magnetic thickness of the second AP pinned layer of the AP pinned layer structure has a magnetic thickness which is greater than a total of the magnetic thicknesses of the first and third AP pinned layers. With this embodiment a current pulse can be conducted through the sensor via the sense current circuit for resetting the magnetic spins of the first and second pinning layers.
Further flexibility in counterbalancing the fields may be accomplished by providing a negative ferromagnetic coupling field on the side of the free layer where the triple AP pinned layer structure is located. This is accomplished by providing a first pinning layer which is composed of platinum manganese (PtMn) and providing particular seed layers for the platinum manganese (PtMn) pinning layer. With this arrangement an appropriately sized first spacer layer will cause the third AP pinned layer next to the first spacer layer to exert a negative ferromagnetic coupling field on the free layer. In all embodiments the net demagnetizing field is counterbalanced by the net sense current field and the net ferromagnetic coupling field.
An object of the present invention is to provide a dual spin valve sensor wherein there is more flexibility in counterbalancing the fields acting on the free layer so as to obtain read signal symmetry.
Another object is to accomplish the foregoing objective as well as minimizing the read gap in order to maximize linear read bit density of the read head.
A further object is to accomplish the foregoing objectives and provide a negative ferromagnetic coupling field when needed in order to obtain read signal symmetry.